An optimal approximation of Rosenblatt sheet by multiple Wiener integrals

Let $Z^{\alpha,\beta}$ be the Rosenblatt sheet with the representation $$Z^{\alpha,\beta}(t,s)=\int^t_0\int^s_0\int^t_0\int^s_0Q^\alpha(t,y_1,y_2)Q^\beta(s,u_1,u_2)B(dy_1,du_1)B(dy_2,du_2)$$ where $B$ is a Brownian sheet, $\frac12<\alpha,\beta<1$, $Q^\alpha$ and $Q^\beta$ are the given kernel. In this paper, we contruct multiple Wiener integrals of the form \begin{align*} \int^t_0\int^s_0\int^t_0\int^s_0&[k_1(y_1,y_2)^{-\frac12\alpha}(u_1,u_2)^{-\frac12\beta}+k_2(y_1\vee y_2)^{\frac12\alpha}(y_1\wedge y_2)^{-\frac12\alpha}|y_1-y_2|^{\alpha-1}\\ &\cdot(u_1\vee u_2)^{\frac12\beta}(u_1\wedge u_2)^{-\frac12\beta}|u_1-u_2|^{\beta-1}]B(dy_1,du_1)B(dy_2,du_2),~~k_1,k_2\geq0, \end{align*} and obtain an optimal approximation of $Z^{\alpha,\beta}(t,s)$.